As the toner for developing a static image, a toner has been usually used, which is prepared by adding a colorant such as carbon black, magnetic powder and a pigment, a charge controlling agent and another additive to a thermoplastic resin, and melting, kneading, crashing, classifying and finishing the mixture.
A method to produce the toner by an emulsion polymerization method or a suspension polymerization method such as those described in Japanese Patent examined Publication nos. 36-10231, 47-518305 and 51-14895 has been developed to solve various problems accompanied with the usual toner for developing the static image prepared by such the crashing method.
However, problems to be solved on the stability of the charging amount and that of the standing up of charging on the occasion of repeating use are found as to the toner produced such the methods.
Japanese Patent Publication Open to Public Inspection, hereinafter referred to as JP O.P.I., No. 10-115952 discloses a method for stabilizing the charging property of the toner by controlling the amount of carboxyl group at the surface of the polymer particle as the raw material of the toner to a specific value for improving such the problems.
However, the above problems cannot be solved as to a high speed writing speed image forming apparatus even when the above method is applied.
Besides, the toner for developing the static image is required to be composed of small particle from the viewpoint for obtaining a high image quality. Recently, a polymerized toner has been actively developed as the method for producing the toner particle with a small diameter. The method for producing the polymerized toner included a method for producing a irregular shaped toner particle by salting out, coagulating and fusion-adhering resin particles and colorant particles according to necessity, and a method by which a radial polymerizable monomer and a colorant are dispersed in an aqueous medium so that the diameter of the dispersed droplet is to be the designated diameter of the toner and the droplet is polymerized by suspension polymerization.
However, the toner produced by the suspension method accompanies a problem that the transferring ability is lowered since the shape of the toner particle is spherical so as to raise the adhesiveness of the toner particle to the image carrying member even though the uniformity of the toner particles can be raised because the spherical toner particle having the uniform surface property can be formed.
Therefore, JP O.P.I. No. 11-194540 discloses a non-spherical particle which is produced by the method in which resin particles polymerized in an aqueous medium containing a surfactant are treated by a coagulation agent in a concentration higher than the critical coagulation concentration of the resin particle and a water miscible organic solvent.
In the foregoing method, precipitation difficultly soluble in water is occurred since the craft point of the surfactant is raised by the presence of a two- or three-valent metal salt even though a toner excellent in the uniformity of the shape and the charging amount and the sharpness of the image formed thereby can be obtained by the use of such the metal salt. The precipitation is remained with the toner particle after the separation of the toner particle from the aqueous medium. As the result of that, problems are caused such as that the fog tends to occur under a high temperature and high humidity condition and the transfer ability tends to be lowered.
Moreover, a suspension polymerized toner and an emulsion associated or coagulated type toner have been be known. The emulsion associated type toner is preferred since the shape of the particle can be easily controlled.
The associated type toner is constituted by an associated particle obtained by coagulating and fusion-adhering of resin particles composed of a polymerizable monomer and another component according to necessity such as colorant by the using a metal salt as a coagulation agent. The metal salt originated from the coagulation agent adhered with the associated particle cannot be easily removed, particularly when the resin particle is composed of a resin derived from a polymerizable monomer containing a polar group such as a carboxyl group, since a large amount of the coagulation agent is necessary for the coagulation/fusion-adhering process.
Thus obtained emulsion associated type toner has high hygroscopicity since the metal salt used as the coagulation agent in the coagulation/fusion-adhering process has high hygroscopicity.
As a result of that, the charging ability of the toner particle is lower when the image formation is performed under a high temperature and high moisture condition compared with that when the image formation is performed under a low temperature and low humidity condition.
Accordingly, a problem such as that a trouble tends to occurred on the dependency of the image density on the environmental condition and the standing up of the charging when the emulsion association type toner is used as the toner for image forming.